1. Field of the Invention
The present invention relates to an image processing apparatus for binary or multi-level digitization of image data, and more particularly to an image processing apparatus for intermediate tone processing of input image data.
2. Related Background Art
For intermediate tone processing in an image processing apparatus such as a facsimile apparatus or a digital copying machine, there are for example proposed an error dispersion method and an average density approximation method.
The former error dispersion method consists of, as disclosed by R. Floyd and L. Steinberg in Adaptive Algorithm for Spatial Gray Scale, SID 75 Digest, pp. 36-37, binary digitizing the multi-level image data of a contemplated pixel into a highest density level or a lowest density level, and adding the error between the thus binarized level and the multi-level image data prior to said binarization to the data of pixels in the vicinity of said contemplated pixel, with a predetermined weighting.
On the other hand, the latter average density approximation method consists of, as disclosed in the Japanese Patent Application Laid-open No. 57-104396, determining the weighted averages of the contemplated pixel and the pixels in the vicinity when said contemplated pixel is binary digitized to black or white, utilizing the binary data already digitized in the vicinity of said contemplated pixel, and binary digitizing the image data of the contemplated pixel, utilizing the average of said two average values as the threshold value.
The above-explained error dispersion method, being designed to correct the error between the input image data and the output image data, is capable of conserving the density of the input image in the output image processing apparatus, and thereby providing an image excellent in the resolving power and the tonal rendition.
However, the correction of the error between the input image data and the output image data requires a large amount of two-dimensional calculations, and the hardware structure becomes inevitably complex because of said amount of calculation.
On the other hand, the average density approximation method, in which the calculation is conducted with data after binary digitizing, is capable of simplifying the hardware structure and achieving a high-speed processing because of the limited amount of processing.
However, since the binary digitizing is conducted by approximation of the contemplated pixel to the average value of an area including said contemplated pixel, the number of tonal levels is limited, and a specific low-frequency texture is generated for an image showing gradual density change, whereby the image quality is deteriorated.
In consideration of the foregoing, the present applicant has proposed, as disclosed in U.S. patent application Ser. Nos. 476,766 and 476,618 (now U.S. Pat. No. 5,121,446) an average density conserving method consisting of determining a single average density, binary digitizing the image utilizing said average density as the threshold value, and adding a correction for the error in binary digitizing.
However, in binary digitizing of an input image with the above-mentioned error dispersion method or the average density conserving method, if the input image has a constant image density continuously over a wide images area as in computer graphic (CG) image, there will result a periodic pattern based on the error dispersing mask on the obtained binary image, or an extremely regular black-and-white pattern in a low or high density image area, so that the image quality is deteriorated.